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IRF540 Won't turn off

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makemyday

New Member
Hello!

I would like to ask for some help in my circuit. What could be the possible reasons why my IRF540 doesn't turn off?
I check the gate voltage of mosfet when I toggle the gate of 2n3904. when I put 5v on the base of BJT the gate voltage of mosfet is 0V and when I switch it to ground the voltage is almost 14V.

Here is my schematic.
1554718742633.png

Thank you!
 

alec_t

Well-Known Member
Most Helpful Member
Welcome to ETO!
Are you sure you have your FET connected correctly? If drain and source were reversed, the body diode inherent in the FET would keep the lamp on.
 

gophert

Well-Known Member
Most Helpful Member
Hello!

I would like to ask for some help in my circuit. What could be the possible reasons why my IRF540 doesn't turn off?
I check the gate voltage of mosfet when I toggle the gate of 2n3904. when I put 5v on the base of BJT the gate voltage of mosfet is 0V and when I switch it to ground the voltage is almost 14V.

Here is my schematic.
View attachment 117547

Thank you!

That is exactly the behavior I would expect.

I) When 0V is applied to the base of 2N3904, there is no current flowing through the 10k ohm resistor so 14v are measured on both ends of that resistor. This means 14 v is applied to the GATE if the MOSFET and the mosfet allows current to flow through your lamp.

II) when 5v is alllied to the BASE of the 2N3904, current flows from collector to emitter and (which must flow through the 10k resistor as well). That means, if you put your volt meter across the emitter and collector of the 2N3904, you'll measure only 0.2v and measuring across the 10k resistor, you'll see about 13.8 volts. That means the V(gs) of the MOSFET is only 0.2 volts so no current will flow through the mosfet - your load will be off.


.
 

makemyday

New Member
Hi alec_t,

Thank you for your response. As i read your comment. I immediately check the mosfet and I found out that the drain and source is shorted. when i tried to replace it with 045n10n a N-channel mosfet. It successfully turn off the lamp. I didnt replace it by Irf540 because i dont have any spare. Thank you!
 

makemyday

New Member
Btw. Should i pull up a 10k resistor in base of 2n3904 to set the mosfet off in initial state?Is that the correct way? Cause I toggle the 2n3904 using a microcontroller and i found out that the mosfet is on for a few seconds and then turn off when the microcrontroller is done with initializing.
 

gophert

Well-Known Member
Most Helpful Member
Btw. Should i pull up a 10k resistor in base of 2n3904 to set the mosfet off in initial state?Is that the correct way? Cause I toggle the 2n3904 using a microcontroller and i found out that the mosfet is on for a few seconds and then turn off when the microcrontroller is done with initializing.
Yes, that is one method, a pull up on the base will keep the NPN on and the MOSFET off when the microcontroller is powering up.
 

gophert

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Most Helpful Member
Also, how many amps is your lamp? Is the mosfet getting warm when the lamp is on?
 

makemyday

New Member
About 1 amp only. I put a heatsink on it. The mosfet is working fine. no heat building up . up to this moment.

Is the 10k pullup and 1k resistor to base is enough?
 

gophert

Well-Known Member
Most Helpful Member
Which exact 045n10n MOSFET are you using? I believe those are rated for some pretty good loads when driven by a 5v microcontroller. That is, you may be able to forego the NPN with your modern MOSFET.
 

gophert

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Most Helpful Member
Your new, modern MOSFET shouldn't make any noticeable heat at one amp. No heat sink required.
 

gophert

Well-Known Member
Most Helpful Member
Is the 10k pullup and 1k resistor to base is enough?

You'll need more resistance or you'll never get the NPN to shut off (depending the sequence of your connection. Post your schematic so I can make fun of it (I mean, check it). The mosfet Gate does not draw any current.
 

makemyday

New Member
I already make a pcb and maybe i will try to remove the npn on the next version.. But for now .. I will try to rework it to set the npn to pull up. As i needed to prove that this circuit is working.

Im using this IPA045n10n3.
Maybe i will stick to this kind of mosfet.
 

gophert

Well-Known Member
Most Helpful Member
I already make a pcb and maybe i will try to remove the npn on the next version.. But for now .. I will try to rework it to set the npn to pull up. As i needed to prove that this circuit is working.

Im using this IPA045n10n3.
Maybe i will stick to this kind of mosfet.
It is much better than the IRF540
 

makemyday

New Member
You'll need more resistance or you'll never get the NPN to shut off (depending the sequence of your connection. Post your schematic so I can make fun of it (I mean, check it). The mosfet Gate does not draw any current.
I will send my schematic later.. I have only an urgent matters to do.
Btw. Thank you for your time helping me. I appreciate a lot.
 

gophert

Well-Known Member
Most Helpful Member
Hi!

Is the resistor value enough to fully turn off and on the 2n3904?
View attachment 117645

Thanks

The values are ok but the connections are wrong.
Actually, in the image in this post, the 10k could be changed to a 100k and 1k changed to 10k.


Connect your resistors like this.
AB0ABA92-5A0F-40DC-BD1F-3C7DCC4CD17F.jpeg
 

Pommie

Well-Known Member
Most Helpful Member
No, that will not work. Put a 10k to 5V so the transistor is held on and a 1k to your pin so the Gnd (pin low) can turn the MOSFET on (transistor off).

Mike.
Edited.
 
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